It is known in the fuel cell art to evaporatively cool fuel cells, thereby deriving the benefit of the heat of vaporization, in contrast with conveying sensible heat to circulating water passing through the cells or coolant passing through coolant plates. Typically, prior approaches to evaporative cooling have taken one of two forms. In a first form, water is abundantly atomized or fogged into the gas stream of one or both of the reactant gases.
The other form of prior approaches utilizes wicking to bring water into the cells. One recent example is shown in U.S. publication 2004/0170878, which is briefly illustrated in FIG. 1 herein. A fuel cell 11 has strips of wicking 12 disposed over a diffusion layer 13 which is in intimate contact with the cathode catalyst in the membrane electrode assembly (MEA) 14. The fuel cell 11 includes an anode 18, which in the subject publication is not involved with cooling. The fuel cell is separated from the next cell in the series 20 by a separator plate 21. A similar separator plate is present, though not shown, on the top of the fuel cell as seen in FIG. 1.
To provide water to the wicking 12, a wicking header 22 extends across the ends of all of the fuel cells on an end thereof which is opposite to the flow of air into the spaces 24 between the wicking 12 that comprise the oxidant reactant gas flow field. Air is supplied by a pump 26 through a manifold 27 to the inlets 28 of each fuel cell.
In FIG. 1, the air flow is exhausted through an outlet header 31 to a condenser 32 which vents the air to exhaust and delivers the condensate to a reservoir 33. Water in the reservoir 33 is conducted to the wicking header 22.
The wicking evaporative cooling described in the aforementioned publication is stated to require external water, from a source outside the fuel cell power plant, since the water generated at the cathode (process water) is said to be insufficient, except at startup, to achieve the necessary cooling. This is also true in an evaporatively cooled fuel cell stack which relies on wicking in U.S. Pat. No. 4,826,741. Therein, 100 cm2 cells have performance of only 0.7-0.8 v at 100-120 mA/cm2 (108-130 A/ft2). Furthermore, the capillary pressure differential along the length of each of the wicks must be greater than the pressure drop along the adjacent air flow field channels in order for there to be a positive wicking velocity, although it is stated that having air flow in the same direction as the flow of water in the wicking means would overcome that problem.
Thus, evaporative cooled fuel cells that rely on wicking require external water, have limited planform size and the performance thereof is limited by small current density.
In order to transport sufficient water to provide the necessary evaporative cooling, from the wicking header 22, located at the perimeter of the cells, to all areas of the cells requiring cooling, the wicking required is considerable, causing each fuel cell to be thicker than is acceptable within the limited volume which is mandated for use in vehicular applications.